The low under-keel clearance affected the manoeuvrability of the vessel. The use of rudder and main engine did not produce the intended result. The propeller thrust was not sufficient to give the ship enough momentum. The low speed over the ground achieved to reach the le de Grce course caused the vessel to drift parallel to that course, and she grounded to the south of the ship channel. During the manoeuvres, the pilot used reference points from landmarks which he was accustomed to using to maintain direction, and he also referred to the rate of turn to monitor the drift of the vessel. However, at night it is more difficult for observers to perceive visual information than in daylight. Thus, when navigating at night, mariners have more difficulty when attempting to identify landmarks. During a swing manoeuvre, the vessel's heading changes continuously; this manoeuvre therefore requires high levels of concentration and skill. Pilots, therefore, must be alert when manoeuvring in such conditions. Their attention must be divided between the progress of the vessel and the observation of visual references, which they must follow continuously. Observation becomes more difficult and further increases the workload, as it is possible to confuse navigation lights, range lights and other illuminated visual references with city lights, which often lie in the background. Navigation by parallel indexing with the aid of radar in stabilized relative presentation mode helps reduce the additional visual workload created by darkness, thereby preventing spatial disorientation. Visual observation of references is one of the primary tasks involved in berthing and unberthing manoeuvres. An aid to navigation at the channel entrance would have allowed a more accurate visual assessment of the drift of the vessel toward the shoal, but Coast Guard advises that care must be taken not to proliferate navigational aids in the area because too many aids could cause interference with normal pilotage.Analysis The low under-keel clearance affected the manoeuvrability of the vessel. The use of rudder and main engine did not produce the intended result. The propeller thrust was not sufficient to give the ship enough momentum. The low speed over the ground achieved to reach the le de Grce course caused the vessel to drift parallel to that course, and she grounded to the south of the ship channel. During the manoeuvres, the pilot used reference points from landmarks which he was accustomed to using to maintain direction, and he also referred to the rate of turn to monitor the drift of the vessel. However, at night it is more difficult for observers to perceive visual information than in daylight. Thus, when navigating at night, mariners have more difficulty when attempting to identify landmarks. During a swing manoeuvre, the vessel's heading changes continuously; this manoeuvre therefore requires high levels of concentration and skill. Pilots, therefore, must be alert when manoeuvring in such conditions. Their attention must be divided between the progress of the vessel and the observation of visual references, which they must follow continuously. Observation becomes more difficult and further increases the workload, as it is possible to confuse navigation lights, range lights and other illuminated visual references with city lights, which often lie in the background. Navigation by parallel indexing with the aid of radar in stabilized relative presentation mode helps reduce the additional visual workload created by darkness, thereby preventing spatial disorientation. Visual observation of references is one of the primary tasks involved in berthing and unberthing manoeuvres. An aid to navigation at the channel entrance would have allowed a more accurate visual assessment of the drift of the vessel toward the shoal, but Coast Guard advises that care must be taken not to proliferate navigational aids in the area because too many aids could cause interference with normal pilotage. Low under-keel clearance affected the manoeuvrability of the vessel. The vessel did not have sufficient momentum to reach the le de Grce course. The use of rudder and main engine did not produce the intended result. Darkness affected the pilot's visual performance. The radar was not used effectively. The Sorel channel entrance is not buoyed.Findings Low under-keel clearance affected the manoeuvrability of the vessel. The vessel did not have sufficient momentum to reach the le de Grce course. The use of rudder and main engine did not produce the intended result. Darkness affected the pilot's visual performance. The radar was not used effectively. The Sorel channel entrance is not buoyed. The grounding of the PONTOPOROS on a shoal south of the channel was due to limited visual observation of landmarks. Navigation was by visual observation only, although it was dark. The low under-keel clearance, which could affect the manoeuvrability of the vessel, was not duly taken into account.Causes and Contributing Factors The grounding of the PONTOPOROS on a shoal south of the channel was due to limited visual observation of landmarks. Navigation was by visual observation only, although it was dark. The low under-keel clearance, which could affect the manoeuvrability of the vessel, was not duly taken into account.